The progress made in recent years in the study of vitamins has been phenomenal in that many disorders previously regarded as of infective origin have been shown to arise from deficiency of some particular item of the diet, which quantitatively is infinitesimal in relation to the total bulk of food taken. Although some five vitamins have been identified and their value established, as yet only one has been isolated in pure form and has acquired the distinction of a chemical formula. This vitamin—fat-soluble D—has a powerful influence in the prevention and cure of rickets; and its relation to dentition and caries of the teeth has been established by careful and prolonged experiment. So far as we know at present vitamin D can be obtained by the body from three different sources. The effects on health of sunlight and ultra-violet rays on the skin have been shown to arise from the capacity of the cells of the sebaceous glands of the skin to manufacture under these influences, from the various sterols they contain, a substance—vitamin D—which is then picked up by the blood-stream and distributed throughout the body. The second source is the fat taken in the diet, butter and cream being particularly rich in this respect. It has also been proved that food exposed to ultra-violet radiation acquires in an added degree the capacity to ward off the ill-effects of vitamin D deficiency. The third and, from the point of view of accurate laboratory control, the most important source is that in the form of ergosterol, a compound related to cholesterol, produced by extraction from yeast and other substances. Ergosterol can be obtained in more or less pure form, and after being subjected for a definite time to irradiation by ultra-violet rays becomes converted into a substance which has all the effects of vitamin D.